(1) Field of the Invention
The present invention relates to a III nitride semiconductor substrate for Epitaxial Lateral Overgrowth (simply referred to as xe2x80x98ELOxe2x80x99, hereinafter), particularly a III nitride semiconductor substrate for ELO preferably usable as a substrate constituting a semiconductor light-emitting element such as a light-emitting diode, or a semiconductor element such as a high frequency or high output electronic device.
(2) Related Art Statement
III nitride films are employed as semiconductor films constituting a semiconductor light-emitting element such as a light-emitting diode, and recently, have caught a lot of attention as semiconductor flints constituting semiconductor elements such as high frequency electronic devices used in e.g. cellular phones.
Such III nitride films are usually formed by MOCVD methods. Concretely, a substrate on which III nitride films are formed is set onto a susceptor installed in a given reactor, and then, heated to 1000xc2x0 C. or over with a heater provided in or out of the susceptor, Thereafter, raw material gases are introduced with a carrier gas into the reactor and supplied onto the substrate
On the substrate, the raw material gases arc dissolved through thermochemical reaction into constituent elements, which are reacted to deposit and form a desired III nitride film on the substrate. The film is required to be low in its dislocation density, so that a semiconductor element constructed by the film attains designed properties.
However, since the melting points of III nitride materials are relatively high, it has been difficult to grow bulk single crystals made of the above III nitride materials. Therefore, there had been no means except that the nitride films are formed by heteroepitaxial growth on a different kind of single crystal substrate such as a sapphire single crystal substrate
Actually, since the lattice constant of a GaN-based III nitride film largely differs from that of a sapphire single crystal substrate, misfit dislocations are created depending on the difference in the lattice constant, at the boundary between the GaN-based III nitride film and the sapphire single crystal substrate. Thereafter, misfit dislocations propagate into the GaN-based III nitride film, increasing the amount of dislocations in the GaN-based III nitride film. Such dislocations due to lattice misfit can be reduced to a certain extent by inserting a buffer layer grown at low temperature between the GaN-based Ill nitride film and the sapphire single crystal substrate.
However, in case of fabricating semiconductor elements required for a high output such as a laser diode or a light-emitting diode with high brightness, a light-detecting device is required for a low dark current, and an electronic device required for a high output at high frequency. The above GaN-based III nitride film was unable to attain desired performances to meet designed properties owing to its high dislocation density.
From the above point of view, an ELO technology has been developed, and a substrate using ELO technology has been developed.
FIG. 1 shows a structure of a conventional III nitride semiconductor substrate for ELO. FIG. 2 shows that a GaN-based III nitride film is formed on the III nitride semiconductor substrate for ELO shown in FIG. 1, by using an ELO technology.
The III nitride semiconductor substrate 5 for ELO shown in FIG. 1 is constructed by sequentially forming the buffer layer 2 made of e.g. GaN grown at low temperature, the underlayer 3 made of e.g. GaN, and the patterned layer 4 made of e.g. SiO2, on the base 1 made of e.g. sapphire single crystal.
As shown in FIG. 2, if the GaN-based III nitride film 6 is formed on the III nitride semiconductor substrate 5 for ELO, dislocations penetrating from the underlayer 3 to the GaN-based III nitride film 6 propagate in the upper direction after laterally propagating as surrounding the pattern 4 as shown by the arrow X1, and propagate in the upper direction only as shown by the arrow X2.
Consequently, the amount of dislocations in the region A of the GaN-based III nitride film 6 above the pattern 4 are reduced. Therefore, the above semiconductor element has good properties because of its constituents"" high crystal quality, utilizing the region A as predetermined constituents of the semiconductor element.
FIG. 3 shows a structure of another example of a conventional III nitride semiconductor substrate for ELO, and FIG. 4 shows that a GaN-based III nitride film is formed by an ELO technology on the III nitride semiconductor substrate for ELO shown in FIG. 3.
The III nitride semiconductor substrate 15 for ELO shown in FIG. 3 is constructed by sequentially forming the buffer layer 12 made of e.g. GaN formed at low temperature, and the underlayer 13 made of e.g. GaN having a concave-convex surface, on the base 11 made of e.g. sapphire single crystal.
As shown in FIG. 4, if GaN-based III nitride film 16 is formed on the HII nitride semiconductor substrate 15 for ELO, misfit dislocations created between the underlayer 13 and the GaN-based III nitride film 16 propagate in the upper direction after laterally propagating from the convex part 13A to the concave part 13B in the under layer 13 as shown by the arrow Y1, and propagate in the upper direction only as shown by the arrow Y2.
Consequently, the amount of dislocations in the region B above the concave part 13B in the GaN-based III nitride film 16 are reduced. Therefore, the above semiconductor element has good properties because of its constituents"" high crystal quality, utilizing the domain B as predetermined constituents of the semiconductor element.
In case of growing the GaN-based III nitride film by means of ELO technology, since there is a region in which dislocations extend above as shown in FIGS. 2 and 4, an inconvenience has been that there is a limitation as to the region usable as a semiconductor element. As a result, if a laser diode, for example, is constructed by the above GaN-based II nitride film, luminous efficiency has been degraded owing to a lot of useless regions not usable as a semiconductor element. causing its yield to lower in photolithography processes.
Furthermore, if a light-emitting diode, for example, is constructed by the above GaN-based III nitride film, there had been an inconvenience that a lot of difference of light-emitting strength exists in the element, causing its light-emitting efficiency to decrease.
Furthermore, in case of utilizing the above GaN-based III nitride film as a substrate for a semiconductor element, it is necessary to form the GaN-based III nitride film comparably thickly, for example, 100 xcexcm and more. However, in case of forming the GaN-based III nitride films by utilizing the III nitride semiconductor substrat 15 for ELO shown in FIGS. 1 or 3, there has been an inconvenience That the surface roughness of the GaN-based III nitride films gets worse considerably, as its thickness becomes larger.
Moreover, in case that a semiconductor element is fabricated by forming a predetermined GaN-based III nitride film on the above III nitride films used as a substrate, the crystal quality of the above GaN-based III nitride film formed on a GaN-based III nitride film semiconductor substrate is deteriorated because of the surface roughness of the GaN-based III nitride film semiconductor substrate. Accordingly, it has been unable to provide desired properties for the above semiconductor element, without carrying out polish processing.
An object of the present invention is to provide a III nitride film semiconductor substrate for ELO, to form a III nitride film having controlled surface roughness not depending on its thickness, as well as enhancing its use efficiency by controlling penetrating dislocations.
In order to accomplish the above object, the present invention relates to a III nitride semiconductor substrate for ELO, characterized in being provided with a predetermined base and a III nitride underlayer including at least Al formed on the base material.
The inventors of the present invention have intensively studied to draw out the above characteristics of ELO technology to the maximum, by controlling an increase of surface roughness in case of forming the above III nitride films thickly, because ELO technology maker it possible to simply improve the crystal quality of the III nitride films by reducing the amount of dislocations in the films As a result, they have found out the following facts. Paying attention to a structure of the III nitride semiconductor substrate for ELO in case of using ELO technology.
As described above, a conventional III nitride semiconductor substrate for ELO is constructed so that a GaN-based buffer layer deposited at low temperature is formed on a sapphire single crystal substrate as a base. The buffer layer makes it possible to grow the above GaN-based underlayer epitaxially, by means of reducing the difference in lattice constant between the sapphire single crystal substrate and the GaN-based underlayer Therefore, the buffer layer is formed at low temperature such as 500-700xc2x0 C., disregarding its crystal quality to some extent.
Accordingly, it is possible to restrain misfit dislocations to some extent, due to the difference in lattice constant. However, it was found out that the GaN-based underlayer""s crystal quality was deteriorated due to a mosaic structure of the buffer layer formed at low temperature. It is presumed that the surface roughness of the GaN-based III nitride film formed on the GaN-based III nitride semiconductor substrate for ELO is caused by the inferior crystal quality of the underlayer constituting the GaN-based III nitride semiconductor substrate for ELO.
Consequently, the present inventors have intensively studied to prevent the underlayer""s crystal quality from deteriorating. As a result, they found out the following facts.
A conventional semiconductor element is constructed by a GaN-based III nitride film as its semiconductor layer constituting the element. Accordingly, it has been naturally thought that an underlayer of a III nitride semiconductor substrate for ELO used in forming a GaN-based III nitride film by ELO technology is constructed by the above GaN-based III nitride.
The present inventors have studied that a semiconductor element is mainly constructed by III nitride films mainly including Al. In the steps of the study, a trial was made that an underlayer of a III nitride semiconductor substrate for ELO is made up of an AlN-based III nitride film. As a result, it was found out that without the above buffer layer formed at low temperature, the AlN-based III nitride film can be epitaxially grown with superior crystal quality on a sapphire single crystal substrate and that it is possible to restrain creation of misfit dislocations at the boundary between the Perborate and the AlN-based III nitride film.
It was found out to be possible to control the roughness of the AlN-based III nitride film formed by an ELO technology on the III nitride semiconductor substrate for ELO, even if the thickness of the AlN-based III nitride film is larger. Furthermore, it also turned out that a GaN-based III nitride film formed on the above underlayer with high crystal quality has the same effect as the AlN-based III nitride.
Moreover, a dislocation density distribution of a III nitride film was investigated. As a result, a dislocation reduction was observed in a region where dislocations penetrated in case of using conventional underlayers. The greater the difference between Al content of the III nitride film and that of the underlayer, the greater the dislocation reduction effect. This is presumably attributed to the principle that compressive stress created in the III nitride film due to the difference in lattice constant has the effect to remove dislocations and to bend dislocation propagation to the transverse direction.
According to the Ill nitride semiconductor substrate for ELO, since the amount of dislocations penetrating through the III nitride film are reduced, its use efficiency increases. Furthermore, in case of the III nitride film being formed thickly on the substrate, it is possible to control the surface roughness of the III nitride film. Consequently, a thick III nitride film preferably usable as a substrate for a semiconductor element can be formed by ELO technology.